It is standard practice to make paper by a process comprising forming a cellulosic suspension, adding a retention system to the suspension, draining the suspension through a screen to form a sheet, and drying the sheet in conventional manner to make the desired paper, which can be a paper board.
The retention system is included in the suspension before drainage in order to improve retention of fibre and/or filler. The retention system can consist of a single addition of polymer in which event the polymer is usually a synthetic polymer of high molecular weight, or the retention system can comprise sequential addition of different retention aids. Before adding a high molecular weight polymer or other retention aid it is known to include low molecular weight polymer, for instance as a wet strength resin or as a pitch control additive. The molecular weight of such polymers is generally too low to give useful retention.
A common retention system consists of high molecular weight (for instance intrinsic viscosity above 4 dl/g) cationic polymer formed from ethylenically unsaturated monomers including, for instance, 10 to 30 mol % cationic monomer. However retention systems are known in which high molecular weight non-ionic polymer or high molecular weight anionic polymer is used.
In EP-A-017353 we describe a retention system for use in "dirty" pulps (having a high cationic demand) comprising bentonite followed by a substantially non-ionic polymer which can be polyethylene oxide or, for instance, polyacrylamide optionally containing small amounts of anionic or cationic groups. Thus one process comprises adding bentonite to the "dirty" suspension and then adding polyethylene oxide.
Another retention system that is sometimes used for dirty suspensions comprises adding water-soluble phenol formaldehyde resin followed by polyethylene oxide, the amount of phenylformaldehyde resin (on a dry basis) generally being substantially greater than the amount of polyethylene oxide.
Advantages of this system are that the materials are relatively inexpensive and that on some dirty pulps it gives very satisfactory retention at low doses. However it suffers from the disadvantage that it frequently gives rather poor results (even on a dirty suspension having high cationic demand) and the reason for the wide variation in results is not fully understood. Another disadvantage is that the phenol formaldehyde resin tends to become increasingly cross linked with time, with the result that performance may deteriorate upon storage of the resin. Another disadvantage is that the molecular weight of water-soluble phenol formaldehyde resins has to be rather low in order to maintain solubility. Increase in the molecular weight of a retention aid would expected to improve retention, but performance may deteriorate when using phenol formaldehyde resins because of reduced solubility.
It would be desirable to provide a retention system that utilises a different phenolic resin that can easily be manufactured to a higher molecular weight while retaining good solubility in water, and that is storage stable, so as to permit more consistent and/or improved retention, especially in dirty pulps.
Another disadvantage with conventional phenol formaldehyde resins is that they may be less effective in acidic suspensions and it would be desirable to be able to use them satisfactorily in such suspensions.